If you have given an ITR and the individual does not make an application to register within a reasonable period of time, you must give them a second invitation.1
If no application is made within a reasonable period of time following the second invitation you must give a third invitation.2
There is no difference in the requirements for the content and delivery of ITRs at the second and third reminder stages.
In practice, the second and third invitations are reminders to the individual to make an application to register. You should consider whether the use of a different delivery method for the second or third invitation could be more effective. For example if you have not received a response to an email ITR, you should consider giving the reminder invitations by post or by hand.
You are not required to send reminder invitations to special category electors.